PURIFICATION AND CHARACTERIZATION OF THE IMMUNOGLOBULIN SWITCH SEQUENCE-SPECIFIC ENDONUCLEASE (ENDO-SR) FROM BOVINE SPLEEN

Mature B lymphocytes are able to specifically alter their Ig isotype e xpression in response to extracellular stimuli via a highly regulated, deletional recombination process called isotype switch recombination. Switch recombination breakpoints predominantly map to large (1-10 kb) , G-rich and highly repetitive switch regions that are located directl y upstream of immunoglobulin heavy-chain constant region genes. Switch region repeat structures vary considerably both within and between sp ecies, but all switch regions contain disproportionate numbers of two pentamer motifs, TGGGN and TGAGC, that are found at or directly adjace nt to most analysed switch junctions. We have recently identified an e ndonuclease activity, Endo-SR, that preferentially cleaves TGGGN and T GAGC switch motifs. We have purified the bovine endonuclease activity to homogeneity and have identified a protein with a molecular weight o f similar to 32 000 that directly correlates with enzyme activity. As discussed in this report, we have found that murine and bovine Endo-SR are preferentially enriched in lymphoid tissue nuclear extracts and t hat both enzymes demonstrate highly similar physical and biochemical c haracteristics. However, each enzyme demonstrates related but distinct ive specificities for consensus and degenerate TGGGN and TGAGC switch pentamer motifs. (C) 1997 Elsevier Science Ltd.